Method of manufacturing zinc oxide



Patented Aug. 29, 1933 recs j 1,924,872 7 7 METHOD OF MANUFACTURING ZINC OXIDE Archibald o. Mason, Highland Parkylll.

No Drawing.

Application March 28, 1932 Serial No. 601,723 g 12 Claims. (01. 23-148) This application maybe consideredin part av continuation of my co-pending applications, Serial Nos. 425,048 and 550,623,,filed January 31, 1930, and July 13, 1931, respectively.

An object of my invention is to provide a method of producing a zinc oxide of hi h purity from low grade high-zinc bearing materials, such as diecasting scrap, zinc-bearing dross, galvanizers dross, die casters dross, and other impure metallic zinc-bearing materials. ,Scrap sheet brass can also be used successfully, if too high a percentage of the zinc is not removed, as copper is found to contaminate the last fraction of zinc.

A further object of my invention is to provide a process whereby such low grade materials high in zinc may be separated to yield not only zinc oxide, which alone is of suiiicient value to enable the operation of my process at a profit, but also the more valuable metals contained in said ma- Step I Where die castings or scrap zinc are contami mated with dirt and oil, the materialis preferably preliminarily melted and run into pigs, the solid impurities being skimmed off.

The first step of my process consists in heating the zinc-bearing metal, preferably in a reverberatory type of furnace or the like, wherein of course, the material is heated from the top, and without agitation, said furnace being heated preferably by oil, although other fuels may obviously be used, to a temperature at which zinc volatilizes. A reducing atmosphere is preferably maintained within the furnace when such carbonaceous fuels are used, in order that free carbon may be deposited in the zinc oxide, for reasons discussed below. It will be understood that various types of furnaces may be used, the principal consideration being that the furnace is preferably of a size capable of holding a melt which will run over-a considerable length of time, say, several days, permitting control through frequent sampling. I prefer a melt of about 50,000 pounds for best results. Another important advantage of having a large melt is that in the course of distillation of the zinc such contaminating metals as tin and lead will tend to settle to the bottom, as a result of which practically pure zinc is distilled off. Such gravitational separation of the metals in the course, of operation is not feasible where a small melt is used. Instead of a gas or oil heated reverberatory furnace, I may use eiiiciently an electric furnace of either the induction or are type.

The zinc vapor is conducted into a combustion chamber preferably formed of firebrick or the like, Where air, controlled by suitable means, as a damper or the like, is admitted at a particular point, so as to insure sudden oxidation. As a result of the sudden oxidation of the zinc, a zinc oxide of fine particle sizeis produced. This is considered of importance, inasmuch as a very fine and smooth zinc oxide is highly desirable as a pigment, pharmaceutical, for other purposes. At a point in the combustion chamber beyond that w ere air is admitted under control, an opening isprovided for inspection. By observing the appearance of the material flowing past this inspection opening, one may determine with a high. degree of accuracy whether or not the proper. amount of air is being admitted to the'combustion chamber, and may vary the admission of air accordingly at the damper.

If desireda pyroineter may be disposedadjacent'the inspection opening to control the damper. Under properly controlled oxidation conditions, the material flowing" past the inspection opening appears as. a denseywhitish smoke with a dull, incandescent glow, caused by the high temperature of the If. too little air is admitted at the damper, the zinc oxidizes too slowly, and as a result the particle size is too large and the cooled, product is yellowish, both undesirable characteristies, If'too much air is admitted, an excessively large quantity of carbon is allowed to remain inthe zincoxide, making the color darker and increasing the dihiculty and expense cf the roastingope" ation, described below.

The zinc oxide in suspon in the stream of furnace gases and air co ted to a baghouse or filtering device of anv other suitable type and is there separated 1"" other satisfactory ma In view of thewid y. composition of the type of alloys bly used by me as raw materials, there is usually present some volatile metals other than nine, these other volatile metals usually being restricted to lead and cadmium. When a run is started, the first zinc which is volatilized will carry off substantially all of the cadmium, inasmuch as cadmium is very volatile. I-fence, the first portion of zinc oxide-.- which willconstitute not more than about 10 per cent of the total product formed in a run-is retnegases in the usual or Til moved from the baghouse and set aside as a separate quality, which may be used in certain industries where the small percentage of cadmium is not objectionable. When cadmium-bearing materials are to be run, the furnace is initially filled and no additions made during the run. On the other hand, if no cadmium is present, additions ray be made to the charge from time to time, maintaining the furnace substantially full.

After distillation of the cadmium, the product will be of a high degree of chemical purity, consisting almost entirely of zinc oxide, and may be used in industries where extremely high chemical purity is essential. Ultimately, the lead concentration remaining in the furnace becomes so high that a fraction thereof is distilled off with the zinc oxide, the percentage of which gradually increases. By this time, a considerable amount of lead will have accumulated, and it may be tapped off very conveniently, since it tends to settle to the bottom of the furnace. If tin is present in the original charge it, also, will to a large extent, settle out alloyed with the lead. This characteristic is considered of importance, inasmuch as it makes possible the production of a large proportion of zinc oxide of very high purity, due to the fact that the zinc rises to the top as the'lead tends to settle to the bottom of the melt, it being understood that the charge is not agitated in order that such segregation may take place.

In general, it is not desired that the zinc oxide be permitted to contain more than 0.25 per cent of lead. A zinc oxide containing such a percentage of lead is considered of a high degree of purity. After the lead content has risen to this figure, it is desirable to tap off the lead from the bottom of the furnace, or empty the entire furnace and start over. However, if desired, a zinc oxide may be formed with higher proportions of lead, which may be used in industries in which the presence of the lead is not undesirable.

The cooled zinc oxide is removed from the baghouse and is of a fine particle size and of a high degree of purity. However, the product is not entirely satisfactory for many purposes because of the fact that it is slightly grayish in color, due to the presence of carbon from the fuel. (Obviously this w ll not occur when an electric furnace is used.) The sudden admission of air into the combustion chamber, as described above, chills the material so suddenly that a small amount of carbon is usually left unburned. Even when of a good color, the material produced by Step I is not altogether desirable because it usually contains occluded gases and some moisture. The specifications for zinc oxide of high purity restrict the proportion of moisture and absorbed or occluded gases, because such impurities raise certain difiiculties in some industries, as, for example, in the paint industry.

Step II The material is, therefore, preferably subjected to a roasting process and apparatus as described in my co-pending application, Serial No. 425,048. Said apparatus consists of an elongated horizontal furnace of the muffle type having an upper surface of firebricl: or the like, over which the zinc oxide produced by Step I is slowly moved and stirred by means of conveyor members. The

zinc oxide is deposited at one end of the upper wall of the furnace and is moved slowly along the length thereof in a series of'piles by means of the conveyor members, the lower layers being kept at a dull red heat, and is continuously stirred and agitated, so that new portions of the material are always being disposed in contact with the heated top surface of the furnace. In this way combustion occurs at the bottom of the piles of zinc oxide, in a limited supply of oxygen, resulting in the burning out of substantially all of the carbon in the material with the formation of carbon monoxide. The presence of carbon monoxide in the heated material is believed to be of value in producing a zinc oxide of good white color. In case yellow oxide is originally produced, due to improper conditions, it be mixed with gray zinc oxide carrying carbon and subjected to the operation of. Step II, and a white product will be formed.

The thus treated zinc oxide is removed from the discharge end of the roasting furnace and is then ready for the market. As a result of the second step described above, the carbon is completely removed from the material, and it is free from moisture and occluded gases and remains so over long periods of time.

The zinc oxide thus produced is fiulfy, ofa highwhite color, and of a very fine particle size, so thatit is extremely smooth and is of a chemical purity of about 99.75 per cent Zn(), or higher.

Hence, this material is ideally adapted for use in paints, as a pharmaceutical, for use in the curing of rubber, and as a pigment in the ceramic industries, etc. etc. wetted by linseed oil and other paint carriers than the ordinary zinc oxide, resulting in fastergrinding, thus permitting the paint manufacturer more accurately to gauge the paint consistency. Re-

moval of occluded gases by my process reduces 1 the danger of livering of paint carriers. Thinning out in the can is eliminated due to the more perfect wetting capacity of my product than" is ordinarily the case with other zinc oxides. When used in rubber manufacture, my product also has many advantages due to its purity, its fine-particle size, and absence of occluded gases and moisture.

The residual orunvolatilized metals from the first step of the processmay be drawn oil" and formed into ingots or the like, and since they contain a high proportion of valuable metals, such as copper, aluminum, etc., may be treated by chemical or electrolytic means for the recovery of said metalsa procedure which would be economically less feasible for the original material which I utilize, due to the high zinc content, but is highly practical after the zinc has been removed.-

It will be apparent, therefore, that, as a result of my invention, materials which are ordinarily considered practically worthless, such as scrap and dross, may be utilized for producing materials of high commercial value, thus saving a portion of the public wealth which has heretofore been substantially a total loss. This is illustrated by the fact that the low grade materials which I utilize, particularly die-cast scrap and die-cast dross, are available in tremendous quantities at an extremely low price, namely, about per cent or less of the price of the zinc oxide which may be produced therefrom. Obviously, the content of more valuable metals, such as copper, aluminum, etc., which are also salvaged by my invention, represents an additional gain. A roasting procedure as outlined above under Step II may be used to advantage for treating various grades of zinc oxide produced by methods other than that described above under Step I, and which are not entirely satisfactory for certain arts.

The product is more readily Thus, for example, a zinc oxide which is produced from an ore may be of a good color but may contain certain occluded gases, such as S02, which render it entirely unsuitable for use in certain arts. Again, a zinc oxide which is produced by other methods may be of a yellowish color, or have certain other disadvantages. All of these materials are greatly improved by subjecting them to the roasting step as described above under Step II, which permanently drives off any moisture or gas from the material, and'changes the color to a brilliant white.

Various modifications, changes and improvements coming within the spirit of my invention will doubtless occur to those skilled in the art. Hence, I do not wish to be limited to the specific procedure described herein, except to the extent indicated in the appended claims, which are to be interpreted as broadly as the state of the art will permit.

I claim as my invention: a

l. A process of treating high-zinc containing alloys, present in die castings, galvanizers dross, and the like, comprising applying surface heat to the material, the latter being in a quiescent state, by means of a fuel which will precipitate free carbon, the conditions being so controlled that practically pure zinc will be distilled oif, oxidizing the zinc vapor to form a zinc oxide containing a small proportion of carbon from the products of combustion, and roasting the resulting material in the persence of a limited supply of oxygen, such as will eliminate the carbon as carbon mon oxide and form a substantially white zinc oxide of high chemical purity.

2. A process as defined in claim 1, wherein the zinc vapor is conducted into a combustion chamber into which a regulated volume of air is admitted so as to insure complete and sudden combustion of the zinc and thus produce a zinc oxide of relatively fine particle size.

3. A method of refining high-zinc alloys of low commercial value present in die castings, dross and the like, comprising subjecting a zincbearing alloy in a furnace, without substantial agitation, to sufficient heat from the top in a reducing atmosphere to melt the mass and volatilize the zinc, conducting the zinc vapor out of said furnace and into a combustion chamber and there completely oxidizing it to form zinc oxide of high chemical purity, and collecting the zinc oxide.

4. A process as defined in claim 3, wherein the oxidation of the zinc is'accomplished by introducing a controlled quantity of air into the combustion chamber so as to insure sudden oxidation of the zinc vapor and thus obtain a zinc oxide of fine particle size.

5. A method as defined in claim 3,. wherein the alloy is heated in a relatively large melt in a reverberatory type of furnace.

6. A method of refining high-zinc alloys of low commercial value present in die castings, dross and the like, comprising heating said material without substantial agitation in a reverberatory type of furnace having a reducing atmosphere at a temperature suificient to melt the mass and volatilize the zinc, conducting the zinc vapor from said furnace into a combustion chamber and there completely oxidizing it to form zinc oxide of high chemical purity, and collecting the zinc oxide.

7. A method of refining high-zinc alloys of low commercial value present in die castings, dross and the like, comprising charging said material into a reverberatory type of furnace, heating said material from the top thereof in a reducing atmosphere at a temperature sufiicient to melt the mass and volatilize the zinc while avoiding substantial agitation and controlling the temperature, so as to avoid distillation of a substantial portion of metals other than zinc, conducting the zinc vapor from said furnace into a combustion chamber and there completely oxidizing it to form zinc oxide of high chemical purity, and collecting the zinc oxide.

8. A process as defined in claim 6, wherein the oxidation of the zinc is accomplished by introducing a controlled quantity of air into the combustion chamber. so as to insure sudden oxidation of the zinc vapor and thus obtain a zinc oxide of fine particle size.

9. A method of refining high-zinc alloys of low commercial value present in die castings, dress and the like, comprising subjecting such metals in a furnace, without agitation, to heat solely from the top sufiicient to melt the mass and volatilize the zinc, conducting the zinc vapor out of said furnace and into a combustion chamber and there completing its oxidation to form zinc oxide of high chemical purity, and collecting the zinc oxide.

10. A method of producing zinc oxide from scrap metals high in zinc contaminated with appreciable quantities of other metals, such as die castings, galvanizers dross and the like, such metals being substantially free from oxides or other chemical compounds of metals, comprising subjecting such metals in a furnace without agitation to heat solely from the top sufiicient to melt the mass and volatilize the zinc, oxidizing the zinc fume thus obtained to produce a zinc oxide of high chemical purity, and collecting the zinc oxide, said furnace being operated with a charge sufficiently large to permit during the course of the distillation substantial gravitational separation of the heavier metals originally contaminating the same.

11. A process as defined in claim 10 wherein the volatilization of the zinc is continued until the concentration in the melt of volatile metals other than zinc becomes so high that the zinc oxide being produced is contaminated with impurities beyond a desired point, and the melt is then poured off.

12. A process as defined in claim 3, wherein the volatilization of the zinc is continued until the concentration in the melt of volatile metals other than zinc becomes so high that the zinc oxide being produced is contaminated with impurities beyond a desired point, and the melt is then poured ofi.

ARCI-IIBALD o. MASON. 

